5.1 How Populations Grow

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• 5.1 How Populations Grow

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THINK ABOUT IT

• In the 1950s, a fish farmer in Florida tossed a
  few plants called hydrilla into a canal. Hydrilla
  was imported from Asia for use in home aquariums
  because it is hardy and adaptable. The few plants
  he tossed in reproduced quickly and kept on
  reproducing. Today, their tangled stems snag
  boats in rivers and overtake habitats native
  water plants and animals are disappearing. Why
  did these plants get so out of control? Is there
  any way to get rid of them?

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THINK ABOUT IT

• Meanwhile, people in New England who fish for a
  living face a different problem. Their catch has
  dropped dramatically, despite hard work and new
  equipment. The cod catch in one recent year was
  3,048 metric tons. Back in 1982, it was 57,200
  metric tonsalmost 19 times higher! Where did all
  the fish go? Can anything be done to increase
  their numbers?

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Describing Populations

• The stories of hydrilla and cod both involve
  dramatic changes in the sizes of populations.
• A population
• is a group of organisms of
• a single species that lives in a
• given area.
• Such as the hydrilla population represented on
  this map.
• Researchers study populations geographic range,
  density and distribution, growth rate, and age
  structure.

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• The current driving force is FERTILITY

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Density and Distribution

• Population density
• refers to the number of individuals per unit
  area.
• Populations of different species often have very
  different densities, even in the same
  environment.
• Ex. A population of ducks in a pond may have a
  low density, while fish and other animals in the
  same pond community may have higher densities.
• Distribution
• refers to how individuals in a population
  are spaced out across the range of the
  populationrandomly, uniformly, or mostly
  concentrated in clumps.

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Density and Distribution

• An example of a population that shows random
  distribution is the purple lupine.
• These wild flowers grow randomly in a field among
  other wildflowers. The dots in the illustration
  represent individual members of a population
  with random distribution.

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Density and Distribution

• An example of a population that shows uniform
  distribution is the king penguin.
• The dots in the illustration represent individual
  members of a population with uniform distribution.

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Density and Distribution

• An example of a population that
• shows clumped distribution is
• the striped catfish.
• These fish organize into tight groups. The dots
  in the illustration represent individual members
  of a population with clumped distribution.

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Growth Rate

• A populations growth rate
• determines whether the population
• size increases, decreases, or stays the same.
• Ex. Hydrilla populations in their native habitats
  tend to stay more or less the same size over
  time. These populations have a growth rate of
  around zero they neither increase nor decrease
  in size.
• The hydrilla population in Florida, by contrast,
  has a high growth ratewhich means that it
  increases in size.
• Populations can also decrease in size, as cod
  populations have been doing. The cod population
  has a negative growth rate.

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Age Structure

• To fully understand a plant or animal population,
  researchers need to know the populations age
  structurethe number of males and females of each
  age a population contains.
• Most plants and animals cannot reproduce until
  they reach a certain age.
• Also, among animals, only females can produce
  offspring.

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Population Growth

• A population will increase or decrease in size
  depending on how many individuals are added to it
  or removed from it.
• The factors that can affect population size are
  the birthrate, death rate, and the rate at which
  individuals enter or leave the population.

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Birthrate and Death Rate

• A population can grow when its birthrate is
  higher than its death rate.
• If the birthrate equals the death rate, the
  population may stay the same size.
• If the death rate is greater than the birthrate,
  the population is likely to shrink.

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Immigration and Emigration

• A population may grow if individuals move into
  its range from elsewhere, a process called
  immigration.
• A population may decrease in size if individuals
  move out of the populations range, a process
  called emigration.

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Human Populations DynamicsPart 2 of 5.1 Types
of Growth
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Exponential Growth

• If you provide a population with all the food
  and space it needs, protect it from predators and
  disease, and remove its waste products, the
  population will grow.
• The population will increase because members of
  the population will be able to produce offspring,
  and after a time, those offspring will produce
  their own offspring.
• Under ideal conditions with unlimited resources,
  a population will grow exponentially.
• In exponential growth, the larger a population
  gets, the faster it grows. The size of each
  generation of offspring will be larger than the
  generation before it.

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Organisms That Reproduce Rapidly

• If you plot the size of this population on a
  graph over time, you get a J-shaped curve that
  rises slowly at first, and then rises faster and
  faster.
• If nothing were to stop this kind of growth, the
  population would become larger and larger, faster
  and faster, until it approached an infinitely
  large size.

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Organisms That Reproduce Slowly

• Many organisms grow and reproduce much more
  slowly than bacteria.
• For example, a female elephant can produce a
  single offspring only every 2 to 4 years. Newborn
  elephants take about 10 years to mature.
• If exponential growth continued and all
  descendants of a single elephant pair survived
  and reproduced, after 750 years there would be
  nearly 20 million elephants!

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Organisms in New Environments

• Sometimes, when an organism is moved to a new
  environment, its population grows exponentially
  for a time.
• When a few European gypsy moths were
  accidentally released from a laboratory near
  Boston, these plant-eating pests spread across
  the northeastern United States within a few
  years.
• In peak years, they devoured the leaves of
  thousands of acres of forest. In some places,
  they formed a living blanket that covered the
  ground, sidewalks, and cars.

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Logistic Growth

• What is logistic growth?
• Logistic growth occurs when a populations growth
  slows and then stops, following a period of
  exponential growth.
• Natural populations dont grow exponentially for
  long.
• Sooner or later, something stops exponential
  growth. What happens?

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Phases of Growth

• Suppose that a few individuals are introduced
  into a real-world environment.
• This graph traces the phases of growth that the
  population goes through.

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Phase 1 Exponential Growth

• After a short time, the population begins to grow
  exponentially.
• During this phase, resources are unlimited, so
  individuals grow and reproduce rapidly.
• Few individuals die, and many offspring are
  produced, so both the
• population size and the rate of growth increase
  more and more rapidly.

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Phase 2 Growth Slows Down.

• In real-world populations, exponential growth
  does not continue for long. At some point, the
  rate of population growth begins to slow down.
• The population still grows, but the rate of
  growth slows down, so the population size
  increases more slowly.

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The Logistic Growth Curve

• This curve has an S-shape that represents what
  is called logistic growth.
• Logistic growth occurs when a populations
  growth slows and then stops, following a period
  of exponential growth.
• Many familiar plant and animal populations
  follow a logistic growth curve.

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The Logistic Growth Curve

• Population growth may slow for several reasons.
• Growth may slow if the populations birthrate
  decreases or the death rate increasesor if
  births fall and deaths rise together.
• In addition, population growth may slow if the
  rate of immigration decreases, the rate of
  emigration increases, or both.

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Carrying Capacity

• When the birthrate and the death rate are the
  same, and when immigration equals emigration,
  population growth stops.
• There is a dotted, horizontal line through the
  region of this graph where population growth
  levels off. The point at which this dotted line
  intersects the y-axis represents the carrying
  capacity.

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Carrying Capacity

• Carrying capacity is the maximum number of
  individuals of a particular species that a
  particular environment can support.
• Once a population reaches the carrying capacity
  of its environment, a variety of factors act to
  stabilize it at that size.